In recent years it has become apparent that treatment of the skin of humans for tanning and/or against acne and/or against psoriasis by using skin treatment devices generating UV light has harmful effects to health, such as skin ageing and skin cancer. In particular, skin cancer is a risk at any age, especially for persons under 18 years old, whose immune system is not completely developed yet, and persons above 25 years old who are prone to increased skin ageing (photoageing), all believed to be induced by overexposure to UV radiation. Hence, from a health point of view it is advisable to limit the exposure to UV light. One way to do this is to limit the exposure time, the second way is to lower the intensity of the UV radiation used. Since a long time regulations have limited the maximum erythemal weighted irradiance allowable for professional skin treatment devices, in particular for tanning, to 0.6 W/m2 for both UV-A (320-400 nm) and UV-B (260-320 nm) which was believed to be a level that involves an acceptable health risk. Erythemal weighted irradiance is the power density of UV-A or UV-B light correlated by the sensitivity of the skin at the specific wave lengths, described in DIN 5031-10. Tanning result is a function of the UV-dose, which is determined by the used UV intensities, the exposure time for a tanning session, and the number of tanning sessions. Health risk can be lowered by using a lower UV dose; however this leads to longer exposure times needed to achieve a comparable tanning result, and a longer exposure time also leads to a higher health risk due to a relatively high cumulative UV dose.
It is an object of the invention to enable a skin treatment device that operates at UV levels that pose a more acceptable health risk. In addition it is an object of the invention to enable a skin treatment device achieving a predetermined grade of skin tanning comparable to known tanning devices using comparable exposure times.